Knock, Knock

There are at least a couple ways to answer your question:

First, Revelation 3:20 is part of a letter written to the Christian church in Laodicea. There is no indication that the people who open the door to Jesus are not saved prior to opening the door, or that opening the door is a metaphor for receiving Christ in salvation. Opening the door does not appear to be a metaphor for salvation, but for obedience. Those who open the door are those who hear the warning of this letter and repent of their sin. The implication from the context of the verse is that these people are already saved.

Second, Revelation 3:20 does not say anything about the process of how one comes to be able to open the door, or about who is able to open the door. Rather, it simply explains the result of opening the door. Even if one were to interpret this verse as speaking of salvation (which I believe would be an incorrect interpretation), the verse still would not provide any information as to how that person came to be able to open the door, whether or not he was regenerated and/or had faith prior to opening the door, etc. Thus, one could understand the verse to be speaking about initial salvation and be a consistent Calvinist. The verse does not refute the idea that a person must be regenerated by the Holy Spirit prior to being able to open the door.

Third (for the sake of argument assuming that this verse is about initial salvation), Calvinism does not require that Jesus forcibly open the door. Rather, Calvinism teaches that the Holy Spirit regenerates man and gives him faith, and that man then responds positively to the gospel in repentance. Metaphorically, opening the door to Jesus would seem to be a better illustration of the response of repentance than of regeneration and reception of faith. Once a person is regenerated and given faith, opening the door is the natural Calvinistic response.